Conventional two-bladed flex-beam rotors have rectangular spar cross-sections (see for example U.S. Pat. No. 4,332,525) such that the spar is broad chordwise to provide high in-plane stiffness. This raises the in-plane natural frequency above the range excited by rotor rotation. The spar is relatively thin and flexible in the out-of-plane direction to reduce bending stresses on the spar root due to coning changes.
There are several problems with the rectangular cross section design. First, the rectangular cross section is structurally inefficient since the center of the beam is lightly loaded in in-plane bending. Second, it is desirable to have a large hole in the center of the rotor through which the rotor head may pass so that the rotor mounting and control is from above. Such a hole in a spar with a rectangular cross section leads to stress concentration around the hole (see U.S. Pat. No. 4,008,980). A two-element cross section is used in U.S. Pat. No. 4,299,538, but it is for the purpose of supporting the entire rotor by the pitch control bearings, a technique that will not work for a teetering rotor.
The recent prior art of rotors has been developed primarily for helicopter applications in which the rotor is powered full time, as opposed to gyrocopters or gyroplanes in which the rotor is not powered or is powered only prior to takeoff. While the spar cross section of the present invention is advantageous for both helicopters and gyroplanes, some of the requirements of the two applications are different. It is desirable for gyroplanes to have jump takeoff capability, in which the rotor is spun up on the ground to a high rotational speed (much higher than is used for takeoff in helicopters) with zero blade angle of attack, then vertical takeoff is performed by increasing the blade angle of attack. The high rotational rate required for jump takeoff requires a rotor with higher in-plane stiffness than is required for helicopters, since the stiffness required to maintain a natural frequency higher than the maximum rotation rate (required in two-bladed rotors) increases with the square of the rotation rate.
To increase the height of the jump takeoff, the rotational inertia of the blade must be increased. One way to increase the rotational inertia is by increasing rotation rate, but the rotation rate is limited by the rotor tip speed which cannot exceed the speed of sound. Another way is with tip weights. However, tip weights that double the rotor rotational inertia also double the in-plane stiffness required to maintain a natural frequency higher than the maximum rotation rate. While some helicopter rotors have tip weights providing inertia to improve their autorotation capability (see U.S. Pat. No. 5,462,409), gyroplane rotors need much heavier tip weights to achieve the inertia required for jump takeoff over obstacles as high as 50 feet.
Tip weights must be in the leading edge of the tip to maintain the correct chordwise balance. Since the rotor blade spar is behind the tip weights, a straight rotor blade centrifugal force exerts a large in-plane bending moment around the spar. The prior art disclosed tip sweep for aerodynamic or acoustic reasons (see U.S. Pat. Nos. 3,721,507, 4,168,939, and 5,332,362) but did not teach the optimal angles of sweep or location of sweep to solve this structural problem.